Filtration mediam support material and filter comprising same

ABSTRACT

The present invention provides a method of preparing a support for a filtration medium comprising passing a sheet of nonwoven fibrous polymeric material having first and second sides through a calender which comprises a nondeformable roll and a resilient roll, the nondeformable roll being maintained at a temperature below the melting temperature of the material, so as to increase the smoothness of the first side of the material which contacted the resilient roll. A filtration medium support material prepared in accordance with the method of the present invention, as well as a filter comprising a filtration medium and that filtration medium support material, are also provided by the present invention.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is a divisional of patent application Ser. No.08/116,901, filed Sep. 3, 1993, now U.S. Pat. No. 5,435,957.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to support materials for filtration mediaand methods for their preparation.

BACKGROUND OF THE INVENTION

Filtration media have been used for the filtration of fine particlesfrom fluids, particularly liquids, for many years. Such filtration mediaare available in a variety of materials to meet particular filtrationrequirements. Microporous membranes, such as those described in U.S.Pat. No. 4,340,479, are particularly well-suited to the filtration offine particulate matter from fluids, especially liquids.

Although microporous membranes possess the ability to remove fineparticles, they unfortunately suffer from a lack of mechanical strength.As a result, one or more supportive materials are often mated with amicroporous membrane in order to provide the membrane with an adequatedegree of mechanical support. This is particularly the case when themicroporous membrane is pleated for use in a filtration element.

Support materials of various compositions and structures have been usedin conjunction with the microporous membranes, including, e.g., nonwovenfibrous materials such as polyesters and polypropylenes. The particularmaterial used in a filtration medium support material preferablypossesses high strength, good edge flow characteristics, and a lowpressure drop across the material. Nonwoven fibrous materials preparedfrom fibers which are at least about 50 microns in diameter provide anexceptional level of performance in each of these areas.

The use of such nonwoven materials as supports for filtration media,especially microporous membranes, however, is not without its problems.In particular, materials prepared from fibers in excess of about 20microns in diameter typically possess a relatively rough or coarsesurface. Thus, when a microporous membrane is mated with such a supportmaterial, the support material, due to its roughness or unevenness, candamage and introduce defects into the membrane, particularly when such amembrane is pleated. One example of such a defect is referred to as"coining." Coining occurs when the support material, upon beingcompressed with a membrane during the preparation of a filtrationelement, leaves a permanent imprint of its surface in the membrane. Themembrane is therefore permanently compressed in the area of the imprint,and this compression results in a local lessening of the membrane'sthickness, which in turn reduces the resolution, or titer reduction, ofthe membrane. In a worst case, a support material will actuallypenetrate the membrane, thereby rendering the membrane defective andallowing undesirable particles to pass through the filter so as tocontaminate the product stream.

It is known that materials which are prepared from fibers which aresmaller in diameter offer superior surface smoothness. However, theadvantage offered by those materials in the area of membranecompatibility is countered by their increased pressure drop, decreasededge flow characteristics, and decreased column strength which isrequired to support the pleats of a pleated membrane. Alternatively,such fine-fibered material can be inserted as a cushioning layer betweenrelatively coarse large-fibered material and a microporous membrane;however, this approach decreases the efficiency of the filtrationelement in several ways. In addition to increasing the pressure dropacross the element, the extra layer adds cost and complexity to theelement. Moreover, the number of pleats that can be prepared from such astructure are reduced. This reduction in the number of pleats reducesboth the surface area available for filtration and the dirt capacity ofthe filtration element.

Thus, there exists a need for a support material which has an acceptablelevel of performance in the areas of pressure drop, strength, and edgeflow characteristics, but which does not unduly damage a filtrationmedium, especially a microporous membrane, when mated therewith to forma filtration element. The present invention provides such a supportmaterial, and a method for preparing the support material, whichprovides such advantages while avoiding the aforesaid disadvantages.These and other advantages of the present invention, as well asadditional inventive features, will be apparent from the description ofthe invention provided herein.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a method for the preparation of a supportfor a filtration medium comprising passing a sheet of nonwoven fibrouspolymeric material having first and second sides through a calenderwhich comprises a nondeformable roll and a resilient roll, thenondeformable roll being maintained at a temperature below the meltingtemperature of the material, so as to increase the smoothness of thefirst side of the material which contacted the resilient roll.

A filtration medium support prepared in accordance with the method ofthe present invention, as well as a filter comprising a filtrationmedium and that filtration medium support, are also provided by thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a method which is able to provide afiltration medium support material which has an acceptable level ofperformance in the areas of pressure drop, strength, and edge flow, butwhich does not unduly damage a high-resolution membrane when matedtherewith. The present inventive method accomplishes this result byprocessing a conventional nonwoven support material, which may beprepared from relatively large diameter fibers, in a manner such thatthe surface smoothness of at least one side of the material is increasedwithout substantially adversely affecting the pressure drop, strength,and edge flow characteristics of the material. The resulting supportmaterial has a smoothness of a support material prepared from smallerdiameter fibers while substantially retaining the pressure drop,strength, and edge flow characteristics of a support material preparedfrom larger diameter fibers.

In particular, the present invention provides a method of preparing asupport material for a filtration medium comprising passing a sheet ofnonwoven fibrous polymeric material having first and second sidesthrough a calender which comprises a nondeformable roll and a resilientroll, the nondeformable roll being maintained at a temperature aboveabout 25° C. and below the melting temperature of the material, so as toincrease the smoothness of the first side of the material whichcontacted the resilient roll.

Any type of nonwoven material suitable for a filtration medium supportmaterial can be processed in accordance with the present inventivemethod with advantageous results. Typically, such nonwoven materialcomprises thermoplastic fibers prepared from polyamides, polyesters,polyolefins, aramides, fluoropolymers, and mixtures thereof. Nonwovenmaterials which comprise thermoplastic fibers prepared frompolypropylene and polyvinylidene fluoride are most typical and can beadvantageously processed in accordance with the present invention.

The fibers used to form the nonwoven material may have any suitablediameter. In particular, the fiber diameter should be sufficiently largeto provide the desired pressure drop, strength, and edge flowcharacteristics, yet small enough to provide the desired smoothnessafter processing in accordance with the present invention. The presentinvention provides for a smoother surface for any given fiber diameter.Thus, fibers with diameters larger than those typically used inconventional supports can be utilized in conjunction with the presentinvention so as to provide a support material with a smoothnessequivalent to that associated with a conventional support materialprepared from fibers of the more typical smaller diameter.Alternatively, an improved smoothness can be attained with a supportmaterial prepared from fibers of the typical diameters used inconventional supports.

Nonwoven material prepared from fibers which are at least about 10microns in diameter is advantageously utilized inasmuch as such asupport material will exhibit acceptable pressure drop and edge flowcharacteristics, as well as an adequate level of smoothness, afterprocessing. Nonwoven material prepared from fibers having a diameter offrom about 15 to about 50 microns is preferably utilized in conjunctionwith the present invention, with a support material prepared from fibershaving a diameter ranging from about 20 to about 40 microns being mostpreferred.

The nonwoven materials that are most preferably used to prepare supportmaterials in accordance with the present inventive method include, forexample, polypropylenes such as Lutrasil® (20 micron fibers, Lutrasil,Freudenberg, Germany) and Typar® (45 micron fibers, Reemay, Old Hickory,Tenn.), polyesters such as Reemay® (20 micron fibers, Reemay, OldHickory, Tenn.), and nylons such as Cerex® (20 micron fibers, FiberwebNorth America, Simpsonville, S.C.).

In accordance with the present invention, the material is passed througha calender, preferably on a continuous basis, which comprises anondeformable roll and a resilient roll. The nondeformable roll can bemanufactured from any material which will not deform upon contact withthe nonwoven material, e.g., any suitable plastic material or metal. Thenondeformable roll preferably is a metal roll, more preferably a steelroll, and most preferably a stainless steel roll, although nickel coatedand chromium plated rolls may also be advantageously utilized. Theresilient roll can consist of any material which provides a degree ofresiliency such that the material, when subjected to the inventiveprocess, is provided with a surface which is smoother than that which itpossessed prior to undergoing the method. Advantageously, the resilientroll has a plastic or synthetic covering, such as silicone rubber,urethane, or nylon, is a fiber roll, such as a cotton-filled roll, or,preferably, has a combined fiber and thermosetting resin covering, suchas one of the Beloit Supertex® coverings.

In carrying out the present inventive method, there are severalvariables that can have a substantial effect upon the final propertiesof the material. Those variables include the temperature of thenondeformable roll, the temperature of the resilient roll, the gapsetting or interference between the two rolls, and the speed of materialthrough the roll-to-roll nip area.

With regard to the temperature variable, the nondeformable and resilientrolls can be maintained at any suitable temperatures. The nondeformableroll is preferably maintained between room temperature, e.g., 20°-25°C., and a temperature below the melting temperature of the supportmaterial. Any further increase would only tend to deform the materialand destroy its structure. Such a material would then exhibit anincrease in at least its pressure drop to the point where the materialwould be rendered effectively unusable for purposes of preparing asuitable filtration element. The temperature of the nondeformable rollis more preferably higher than 25° C., e.g., at least about 50° C. or100° C., and most preferably at least as high as the glass transitiontemperature of the support material, in order to impart the desireddegree of smoothness to the support material. Accordingly, thetemperature of the nondeformable roll is most preferably maintained at atemperature between the glass transition temperature and the meltingtemperature of the support material. The nondeformable roll may beheated by any suitable means.

The resilient roll is preferably not directly heated, although it may bewarmed to some extent due to the proximity or surface contact of theresilient roll with the nondeformable roll. Both sides of the materialare changed, i.e., smoothed, when subjected to the present inventivemethod, and the side that contacts the resilient roll is the mostimproved as to smoothness.

The gap setting or interference between the two rolls is maintained tocompress, but not crush, the material. Excessive compression anddeformation of the material will increase the pressure drop and reducethe edge flow rate such that the material is no longer suitable for usein a filtration element. In contrast, insufficient compression will notimpart the desired degree of smoothness to the support material.Sufficient compression of the material may be maintained by conventionalroll-to-roll gap setting methods in conjunction with the pressureloading of one roll into the other. The nondeformable roll and theresilient roll may contact one another or may be advantageouslyseparated, e.g., by about 5% to about 80% of the thickness of thematerial prior to passage through the calender. One of the nondeformableand resilient rolls is preferably pressure loaded into the other roll ata pressure of at least about 50 pounds per linear inch. The pressureloading more preferably ranges from about 50 to about 3,000 pounds perlinear inch or more, with the most preferred range being about 150 toabout 800 pounds per linear inch.

The rate at which the material passes through the calender also affectsthe properties of the processed material. If the material passes throughthe calender too quickly, the smoothness of the material is notincreased to the most desirable extent. This is particularly the casewhen the nondeformable roll is heated, resulting in an inadequate amountof heat being transferred to the material if the material passes throughthe calender too quickly. Of course, when the nondeformable roll isheated, running the material through the calender at too slow a rate mayresult in over-heating of the material, resulting in the same type ofdamage to the material that is experienced when the temperature of thenondeformable roll is too high. Advantageously, the material passesthrough the calender at a rate of about 5 feet/min to about 500feet/min, preferably at a rate of about 5 feet/min to about 100feet/min, and most preferably at a rate of about 10 feet/min to about100 feet/min.

After the material exits the calender, an increase in the smoothness ofthe side of the material which contacted the resilient roll isexperienced. Advantageously, this change in smoothness of one side ofthe material after calendering as compared to the initial smoothnessresults in a decrease by at least about 15% in the force required toslide the material against itself (in the manner described more fully inthe examples below). Preferably the decrease in the sliding force is atleast about 25%, more preferably at least about 40%, and most preferablyat least about 50%. In quantitative terms, the sliding force ispreferably no more than about 20 ounces, more preferably no more thanabout 15 ounces, and most preferably no more than about 12 ounces, asdetermined by the force required to slide the material against itself.

The permeability of the material may be affected as a result ofsubjecting it to the method of the present invention. Permeability is ameasure of the ability of a fluid to penetrate through a medium, andoften it is measured as the difference in pressure across the mediumwhen a fluid (i.e., liquid or gas) is passed through the medium at aknown rate. This measurement is known as delta P, whereas the flowthrough the medium under a constant driving pressure is commonly knownas the Frazier number. The delta P or Frazier number of the material ispreferably essentially unaffected by the present inventive method or atleast not so adversely affected as to interfere with the intended use ofthe material as a filtration medium support material.

If desired, a material which has already been subjected to the method ofthe present invention can be turned over and passed through the calendera second time. This provides a material in which both sides have anincreased degree of surface smoothness as compared to theirpreprocessing smoothness.

Support materials prepared using the method of the present invention canbe mated with any suitable filtration medium, e.g., microporousmembrane, with the smooth side contacting the filtration medium, to forma filtration element. Such an element can be used in a flat form or,preferably, is configured into a pleated form. When both sides of amaterial have been subjected to the method of the present invention,such material can be advantageously used to prepare a dual-layeredfiltration element. In such an element, two filtration media are placedinto a cartridge, with the aforesaid two-sided smoothed support materiallocated therebetween. If desired, two additional pieces of supportmaterial, also prepared pursuant to the present invention, can be matedto the upstream side of the first filtration medium and the downstreamside of the second filtration medium, with their smooth sides contactingthe filtration media. Such an arrangement can be pleated and insertedinto a device, e.g., a cartridge, for use in any suitable filtrationprocess.

The following examples further illustrate the present invention but, ofcourse, should not be construed as in any way limiting its scope asdefined by the claims.

EXAMPLE 1

This example illustrates the preparation of a support material inaccordance with the method of the present invention.

Typar® 3121 polypropylene fabric was passed through a calender at therate of 20 ft/min. Such fabric has two distinct surfaces, which arereferred to herein as the inside and outside surfaces. The calenderconsisted of a chrome-plated steel roll, 6 inches in diameter and 12inches wide, heated to 135° C. and a Supertex® roll, 9 inches indiameter and 12 inches wide, which was not heated. There was no gapbetween the rolls, and the pressure loading of one roll into the otherwas 1500 psig. Some of the fabric was passed through the calender suchthat the inside surface contacted the Supertex® roll, while other of thefabric was passed through the calender such that the outside surfacecontacted the Supertex® roll.

The properties of the fabric before and after processing are set forthbelow, wherein the side of the fabric sample contacting the Supertex®roll is identified in parentheses adjacent the processed fabricindication. The determination of fabric smoothness was made by measuringthe force in ounces to slide a fabric surface against itself. Theprocedure consisted of securely attaching a fabric sample of known sizeto a standard stainless steel sled (15 in²) providing 3.5 oz/in² load,and then measuring the force to move the sled across the same size ofthe fabric being tested. The testing was repeated in several areas ofthe fabric, and the average of the test results reported as the slideforce.

    ______________________________________                                                              Processed Processed                                                           Fabric    Fabric                                                   Original   (Inside   (Outside                                      Property   Fabric     Surface)  Surface)                                      ______________________________________                                        Weight (oz/yd.sup.2)                                                                     1.15       1.18      1.18                                          Thickness (mils)                                                                         7.8        4.8       4.5                                           ΔP8 (inch H.sub.2 O)                                                               0.03       0.07      0.08                                          Inside/Inside                                                                            22.0       16.0      --                                            Slide Force (oz)                                                              Outside/Outside                                                                          19.0       --        16.0                                          Slide Force (oz)                                                              ______________________________________                                    

The present invention resulted in an improvement of smoothness of theinside surface of the support material of about 27% as indicated in thereduction in slide force. The outside surface of the support materialwas improved about 16% by the present invention, as indicated in thereduction in slide force.

EXAMPLE 2

This example further illustrates the preparation of a support materialin accordance with the method of the present invention.

Polyvinylidene fluoride (PVDF) fabric was passed through a calender atthe rate of 10 ft/min. Such fabric has two distinct surfaces, which arereferred to herein as the inside and outside surfaces. The calenderconsisted of a chrome-plated steel roll heated to 160° C. and aSupertex® roll which was not heated. There was no gap between the rolls,and the pressure loading of one roll into the other was 2500 psig. Someof the fabric was passed through the calender such that the insidesurface contacted the Supertex® roll, while other of the fabric waspassed through the calender such that the outside surface contacted theSupertex® roll.

The properties of the fabric before and after processing are set forthbelow, wherein the side of the fabric sample contacting the Supertex®roll is identified in parentheses adjacent the processed fabricindication. The determination of fabric smoothness was made in the samemanner described in Example 1.

    ______________________________________                                                              Processed Processed                                                           Fabric    Fabric                                                   Original   (Inside   (Outside                                      Property   Fabric     Surface)  Surface)                                      ______________________________________                                        Weight (oz/yd.sup.2)                                                                     2.1        2.1       2.5                                           Thickness (mils)                                                                         11.2       4.0       4.0                                           ΔP8 (inch H.sub.2 O)                                                               0.05        0.28      0.38                                         Inside/Inside                                                                            40.8       8.6       --                                            Slide Force (oz)                                                              Outside/Outside                                                                          33.6       --        11.4                                          Slide Force (oz)                                                              ______________________________________                                    

The present invention resulted in an improvement of smoothness of theinside surface of the support material of about 79% as indicated in thereduction in slide force. The outside surface of the support materialwas improved about 66% by the present invention, as indicated in thereduction in slide force.

All of the references cited herein are hereby incorporated in theirentireties by reference.

While this invention has been described with an emphasis upon apreferred embodiment, it will be obvious to those of ordinary skill inthe art that variations of the preferred products and processes may beused and that it is intended that the invention may be practicedotherwise than as specifically described herein. Accordingly, thisinvention includes all modifications encompassed within the spirit andscope of the invention as defined by the following claims.

What is claimed is:
 1. A filtration medium support material comprising asheet of nonwoven fibrous polymeric material having first and secondsides, said nonwoven fibrous polymeric material having a sliding forceagainst itself of no more than about 15 ounces on both sides thereof. 2.The filtration medium support material of claim 1, wherein the fiberswhich comprise said nonwoven fibrous polymeric material are at leastabout 10 microns in diameter.
 3. The filtration medium support materialof claim 3, wherein the fibers which comprise said nonwoven fibrouspolymeric material range from about 15 to about 50 microns in diameter.4. The filtration medium support material of claim 3, wherein the fiberswhich comprise said nonwoven fibrous polymeric material arepolypropylene.
 5. The filtration medium support material of claim 4,wherein said nonwoven fibrous polymeric material has a sliding forceagainst itself of no more than about 12 ounces on both sides thereof. 6.A filter comprising a filtration medium and the filtration mediumsupport material of claim
 5. 7. The filtration medium support materialof claim 3, wherein the fibers which comprise said nonwoven fibrouspolymeric material are polyvinylidene fluoride.
 8. The filtration mediumsupport material of claim 7, wherein said nonwoven fibrous polymericmaterial has a sliding force against itself of no more than about 12ounces on both sides thereof.
 9. A filter comprising a filtration mediumand the filtration medium support material of claim
 8. 10. Thefiltration medium support material of claim 1, wherein said nonwovenfibrous polymeric material has a sliding force against itself of no morethan about 12 ounces on both sides thereof.
 11. A filter comprising afiltration medium and the filtration medium support material of claim10.
 12. The filtration medium support material of claim 1, wherein thefibers which comprise said nonwoven fibrous polymeric material areselected from the group consisting of polyamides, polyesters,polyolefins, aramides, fluoropolymers, and mixtures thereof.
 13. Thefiltration medium support material of claim 12, wherein the fibers whichcomprise said nonwoven fibrous polymeric material are selected from thegroup consisting of polypropylene and polyvinylidene fluoride.
 14. Afilter comprising a filtration medium and the filtration medium supportmaterial of claim
 1. 15. A filter comprising a filtration medium and afiltration medium support material mated thereto, wherein saidfiltration medium support material comprises a sheet of nonwoven fibrouspolymeric material having first and second sides and having a slidingforce against itself of no more than about 15 ounces on at least oneside thereof which contacts said filtration medium.
 16. The filter ofclaim 15, wherein the fibers which comprise said nonwoven fibrouspolymeric material are at least about 10 microns in diameter.
 17. Thefilter of claim 16, wherein the fibers which comprise said nonwovenfibrous polymeric material range from about 15 to about 50 microns indiameter.
 18. The filter of claim 17, wherein the fibers which comprisesaid nonwoven fibrous polymeric material are polypropylene.
 19. Thefilter of claim 18, wherein said nonwoven fibrous polymeric material hasa sliding force against itself of no more than about 12 ounces on bothsides thereof.
 20. The filter of claim 18, wherein said filtrationmedium is a membrane.
 21. The filter of claim 17, wherein the fiberswhich comprise said nonwoven fibrous polymeric material arepolyvinylidene fluoride.
 22. The filter of claim 21, wherein saidnonwoven fibrous polymeric material has a sliding force against itselfof no more than about 12 ounces on both sides thereof.
 23. The filter ofclaim 21, wherein said filtration medium is a membrane.
 24. The filterof claim 17, wherein said filtration medium is a membrane.
 25. Thefilter of claim 16, wherein said filtration medium is a membrane. 26.The filter of claim 15, wherein the fibers which comprise said nonwovenfibrous polymeric material are selected from the group consisting ofpolyamides, polyesters, polyolefins, aramides, fluoropolymers, andmixtures thereof.
 27. The filter of claim 26, wherein the fibers whichcomprise said nonwoven fibrous polymeric material are selected from thegroup consisting of polypropylene and polyvinylidene fuoride.
 28. Thefilter of claim 27, wherein said filtration medium is a membrane. 29.The filter of claim 26, wherein said filtration medium is a membrane.30. The filter of claim 15, wherein said nonwoven fibrous polymericmaterial has a sliding force against itself of no more than about 15ounces on both sides thereof.
 31. The filter of claim 30, wherein saidnonwoven fibrous polymeric material has a sliding force against itselfof no more than about 12 ounces on both sides thereof.
 32. The filter ofclaim 15, wherein said nonwoven fibrous polymeric material has a slidingforce against itself of no more than about 12 ounces on at least oneside thereof which contacts said filtration medium.
 33. The filter ofclaim 32, wherein said filtration medium is a membrane.
 34. The filterof claim 15, wherein said filtration medium is a membrane.